tl;dr; Only Amazon really benefits, buyers will eat the cost and their experience might be worse. Smaller sellers will suffer the most.
Mostly it let's Amazon hire fewer people in support department - they don't have to deal with the occasional refused return ("I dropped this glass item on tiles and it broke into a million pieces, I'd like my money back. Do I have to ship it back to you?" - paraphrased actual return request)
The only other plus side I see is that customers can ship the return immediately. Not much of a difference as sellers are now required to respond within 12h (7 days a week, including holidays) and if they fail to more than 5-10% of the time (I don't remember what the exact target is now) they risk having their Amazon account closed.
From consumer point of view it's not a good thing:
- it offers no new customer protection. sellers are already required to accept those returns (unless customer shoots themselves in the foot - see example above) and provide a prepaid shipping label (in most cases). Amazon enforces that
- it removes the option seller has to accept a return that is out of policy
- it increases the number of unnecessary returns (and having to reorder). Often a one sentence explanation/hint is enough
- it doesn't make the cost of shipping to customer refundable, unless the customer lies about the reason why he's returning the item. Right now those false claims account for at least 3/4 of all returns (source: my wife runs a business that does part of the sales through Amazon, other sellers we know having the same problem)
- it generates extra costs for sellers. that cost is ultimately paid by buyers
From seller point of view it's not a good thing:
- makes fraud easier. For a larger seller that's simply a cost of doing business that will be shifted to buyers anyway. For a small seller it can be end of business if for example a Chinese company that copied their product decides to make a couple dozen fraudulent returns (not that uncommon)
- removes an option to use your own cheaper shipping label - even sending just a few hundred package a month gets you really nice discounts
- removes an option to use your own shipping label that makes the return safer (like extra insurance or using a courier). ANYTHING customer ships back makes Amazon consider it a valid return - even if it's damaged, something completely different, etc the seller will have to provide a refund. It's next to impossible to stop that refund
- removes an option to receive feedback from the customer making the return to improve in the future